Inorganic glasses are well known as composite fillers or fibers for a variety of applications. Specifically, phosphate glasses are known to be absorbable and have been considered, in combination with slow-absorbing hydrophobic matrices, for use in the formation of implantable medical devices.
It is believed that medical plates or screws, such as are employed in the internal fixation of fractured rigid bones, which are formed of high modulus absorbable phosphate glass fibers in an appropriate matrix will yield a composite of a desired level of stiffness which retains measurable mechanical strength at three to six months and fully absorbs in one to three years, thereby maintaining mechanical integrity as the bone heals and absorbing within a reasonable period. However, it has been recognized that there are two basic shortcomings of such a glass-reinforced system, namely, the unacceptably fast absorption and strength loss profile of the glass fibers and poor interface or adhesion between the inorganic glass fibers and the organic matrix.
It has been shown that high temperature annealing of iron-containing phosphate-glass fibers is a promising approach to prolong the strength retention of the fibers and, possibly, the absorbable composites thereof.
To address the fiber-matrix interface, recent investigators have attempted, with limited success, the use of coupling agents, a successful strategy in thermosetting nonabsorbable composites, to improve the interfacial adhesion in absorbable composites of thermoplastic matrices. This created a need for a new approach to creating a novel interface in absorbable glass-reinforced composites which can also be applicable to the more traditional, industrially important nonabsorbable composites.
Nonabsorbable fiber-reinforced composites are used widely in structural industrial applications as well as biomedical applications (e.g., dental and orthopedic). However, as in all composites, the interface between the matrix and reinforcing filler, be it a fiber or powder, has a great effect on the effectiveness of the latter in imparting desirable properties. Toward increasing the interaction or adhesion of glass fillers to organic matrices for composites in general, the use of silane coupling agents has been known for a few decades and was associated with variable degrees of success.
For composites of thermoplastic polymers with silicate glass filler in addition to the silane coupling agents, silicate glass has been surface modified to provide covalently linked hydroxylic groups which were allowed to condense with oligomeric carbonate molecules to place grafts on the glass surface. It is claimed that having these grafts is responsible for achieving a modifying interface and increases the adhesion between the filler and matrix. In effect, such treatment is reported to improve the mechanical properties of the polycarbonate composites. However, this process of surface modification is limited by (a) kinetics of the condensation reaction; (b) the type and chain length of the organic polymer and availability of reactive end groups for condensing with the hydroxyl group; (c) use of high temperature to achieve condensation and possible formation of degradation products; and (d) formation of condensation by-products that need to be removed. Thus, a novel approach for grafting organic chains onto glass surface that features none of the disadvantages associated with the polycarbonate coupling (or condensation) was conceived and is the subject of this invention.
Thus, it is an object of the present invention to provide a surface modified glass having improved compatibility with a variety of composite matrices.
It is a further object of the present invention to provide a method for the surface modification of both absorbable and nonabsorbable glasses to allow chemical abridging of the inorganic fibers to the organic matrix.
It is yet another object of the present invention to provide a novel phosphate-based glass which has a reduced absorption rate.